Episode 313: Polarisation
This episode requires students to develop their idea of electromagnetic radiation. Since they
cannot see either the wave nature of light or the molecular structure of Polaroid, they will have to
take some of this on trust. You can establish the basic ideas using analogies. The approach is
non-mathematical.
Summary
Student experiment: Using polarising filters to observe polarisation effects. (5 minutes)
Discussion: A simple explanation of polarisation. (15 minutes)
Demonstration: Polarisation of light, microwaves and radio waves. (30 minutes)
Demonstration: Polarisation of light by scattering. (10 minutes)
Student questions: Questions on polarisation. (30 minutes)
Discussion: A summary. (5 minutes)
Student activity: Aerials and polarisation (30 minutes)
Student activity: Solutions may rotate polarisation. (30 minutes)
Student experiment:
Using polarising filters to observe polarisation effects
Provide each student with two Polaroid filters. Ask them to look through them at light sources (a
lamp, the sky, (particularly at 900 to the Sun), etc.). Try one filter, then two. Rotate one relative to
the other.
(It is helpful if the filters are rectangular rather than square, or marked in some way to help
students keep track of the orientation.)
(Diagram: resourcefulphysics.org)
They should notice that one filter reduces the intensity of the light. A second can cut it out
completely, if correctly oriented.
Discussion:
A simple explanation of polarisation
Check that your students can recall the difference between transverse and longitudinal waves.
1
Point out that most wave properties are shared by both transverse and longitudinal waves, but
there is one that distinguishes between the two – polarisation. Because this can only happen with
transverse waves, it has given us useful information about the nature of waves.
(Diagram: resourcefulphysics.org)
Show this diagram; the blue wave is polarised in a vertical plane, and so can pass through a
vertical slot. The red wave is polarised in the horizontal plane, and cannot pass through.
(Note that it is better to talk about ‘plane-polarised waves’, rather than simply ‘polarised’, as this
will save confusion later.)
Discuss why longitudinal waves cannot be polarised.
Can students relate this to their observations with the Polaroid filters? Here is a simple
explanation of how Polaroid filters work – use this if you think your students want a bit more
explanation:
You will have to state that light (and other electromagnetic radiations) consists of oscillating
electric and magnetic fields. Polaroid is a type of plastic; its molecules are long chains, oriented
parallel to one another. There are electrons that are free to run up and down the chains.
When the oscillating electric field is vertical, and the chains are vertical, the electrons are caused
to move up and down with the same frequency. (The chains are like miniature aerials, absorbing
the radiation.) At the same time, the electrons re-emit the radiation in all directions, and the result
is that not much radiation passes straight through.
If the polymer chains are at right angles to the electric field, the electrons cannot move very far
and thus do not absorb much energy from the wave, so it passes through. At any other angle, it is
the component of the electric field perpendicular to the chains which passes through; this
explains why the light dims as you rotate the filters.
2
Demonstration:
Polarisation of light, microwaves and radio waves
Here you can show that light, microwaves and radio waves can all be polarised.
TAP 313-1: Polarisation of waves
Demonstration:
Polarisation of light by scattering
TAP 313-2: Polarisation by scattering
When light passes through a cloudy liquid, some is scattered. The scattered light is polarised.
Set this up in advance; show it briefly, and invite students to look at the transmitted and scattered
light through polarising filters during the rest of the episode.
Use this diagram to help explain why scattered light is polarised.
scattered light
scattered light
(Diagram: resourcefulphysics.org)
TAP 313-3: Polarisation of light by scattering
Student questions:
Questions on polarisation
It will help students if you explain that the length of an aerial is often one-quarter or one-half
wavelength.
Make a selection of questions that you feel are relevant to your students.
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TAP 313-4: Polarisation in practice
Discussion:
A summary
Summarise the ideas that you have been looking at: we know that electromagnetic waves are
transverse because they can be polarised. Sound cannot be polarised, and so must be
longitudinal. Emphasise that polarisation is good evidence for the wave nature of light; reflection
and refraction can both be explained without recourse to the idea of waves. Later, students will
see that interference and diffraction are both also characteristic of waves rather than particles.
Student activity:
Aerials and polarisation
This could be a home experiment. It will not be possible for everyone to see every type of aerial
but observations could be pooled and discussed.
In a radio, the ferrite rod increases the magnetic field and so should be parallel to the magnetic
field of the em radio wave(some specifications mention the alignment of aerials).
TAP 313-5: Home experiments with radio and television signals
Student activity:
Solutions may rotate polarisation
Polarimeters and the rotating effect of sugar; used in the sweet industry.
This could form the basis of an investigation.
TAP 313-6: Polarimetry
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(Diagram: resourcefulphysics.org)
TAP 313 - 1: Polarisation of waves
How does polarisation work?
Many kinds of polariser filter out waves, leaving only those with a polarisation along the direction
allowed by the polariser. Any kind of transverse waves can be plane polarised.
You will need

three polarising slots (hardboard, see below)

a 3 or 4 m length of rubber pressure tubing that can be threaded through the slots

a G clamp, retort stand, boss and clamp to secure one end of the rubber tubing

three polarisers, optical, 50mm  50mm, to place on an overhead projector

plastic moulded transparent ruler with notch

microwave transmitter

microwave receiver

polarising grille, microwave

audio amplifier

loudspeaker

1GHz UHF oscillator (30 cm kit) with dipole transmitter / receiver and rod to rotate plane
of polarisation

microvoltmeter as detector for rectified 1GHz waves
Seeing polarisation
You can see polarisation in:

waves on a rope

light waves

3cm microwaves

1GHz UHF radio waves
Waves on a rope
You need three hardboard sheets about 0.5 m  0.5 m, with a centrally cut slot about 300 mm
long and 15 mm wide, marked with arrows showing the permitted direction of vibration.
Visibly polarised waves on a rope can be passed or blocked by a mechanical 'polarising filter',
consisting just of a board with a slot cut in it.
If the slot lies along the direction of vibration, the wave gets through. If the slot lies across the
direction of vibration, the wave is stopped (or reflected). If the slot lies at some in-between angle,
some of the wave gets through, but not all.
If the slot is at an angle  to the direction of vibration of the incoming wave then only a component
A cos  of the original wave amplitude A is transmitted. The component A sin  perpendicular to
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the slot is blocked. The emerging wave is polarised parallel to the slot: the direction of
polarisation has effectively been rotated, and the amplitude of the wave has been reduced.
When the permitted direction of vibration or polarisation of the filter is parallel to the direction of
the polarisation of the wave, it is transmitted by the filter.
When the permitted direction of vibration or polarisation of the filter is parallel to the direction
of the polarisation of the wave, it is transmitted by the filter.
When the permitted direction of vibration or polarisation of the filter is perpendicular to the
direction of the polarisation of the wave, it is absorbed or reflected but not transmitted.
Polarising light
Light from a hot filament lamp is unpolarised (or rather, is emitted in randomly changing directions
of polarisation). A polarising filter made of polaroid polarises the light. The filter passes
components vibrating parallel to a special direction, and removes components vibrating
perpendicular to this direction, so roughly halving the intensity of the light. A second filter with its
special direction at right angles to that of the first will cut out almost all the light.
If you look through a polarising filter at light reflected from glass surfaces, you will often find that it
is at least partly polarised. Just rotate the filter to see if there is a reduction in brightness. Try the
same with the blue sky on a sunny day.
A third polariser put between two 'crossed' filters can let some light through. The middle filter
rotates the direction of polarisation of light from the first filter so that some of it now gets through
the second one. This is put to use in visualising stress patterns in materials.
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1. parallel polaroid filters
2. crossed polaroid filters
3. rotation by intermediate filter
A piece of acetate with a notch cut in it and bent to stress the material at the notch will show
stress patterns if placed between crossed polarising filters.
notch
acetate
strip
crossed
polaroid
filters
Polarised 3 cm microwaves
You can easily show that these microwaves are polarised when they are emitted. This can be
done just by putting a receiver in front of the transmitter, and then rotating either around the
direction between them. When they are 'crossed' (at right angles) the signal reception drops to
zero.
A polarising filter can be made of a grille of metal wires. Held with its wires parallel to the direction
of polarisation (the direction of oscillation of the electric field) it does not pass any signal. Held
with the grille at right angles to the direction of polarisation, the microwaves get through.
Safety
It is important to check that the power supplies are electrically safe. Those transmitters using a
Klystron oscillator require about 300 V at a hazardous current. Ensure that all high voltage
connectors are a safety pattern.
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receiver with
audio amplifier
transmitter of 3cm
microwaves
to power supply
rotation of either device demonstrates the polarisation
a polarising grill has its permitted
direction of vibration of the electric
vector perpendicular to the wires
orientation to
pass through
receiver
transmitter
zero transmission for
this filter orientation
rotation of filter allows
a component to pass
This seems to be contradictory behaviour compared to the ‘mechanical filter’. When the grill is
parallel to the direction of polarisation, the free electrons in the metal are accelerated by the
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electric field in the em wave, thus absorbing energy from the wave. The energy is re-radiated in
all directions (the metal acts like an aerial), so the wave travelling in the onward direction as if it
had passed through the grill is very weak.
Plane polarised waves meeting a grill with wire at right angles to the direction of vibration do not
have much energy absorbed by the free electrons in the metal. They can only be moved for a
short distance, so most of the energy in the wave passes onwards.
Polarised 1 GHz UHF radio waves
Radio waves are transmitted by a dipole aerial, much like that used in rooftop television aerials.
The direction of polarisation is parallel to the dipole rods. A receiving dipole picks up the waves
when it is parallel to the transmitter, but not if it is held 'crossed'.
A metal rod about 15 cm long will act as a polarising filter and can be used to rotate the angle of
polarisation.
receiving dipole
metal rod rotated between dipoles
wires to
galvanometer
wires
to 1 GHz oscillator
components can be rotated to demonstrate polarisation
and rotation of the direction of the electric oscillation
transmitting dipole
You have now:
1.
Seen polarisation in several different cases.
2.
Understood that transverse waves show polarisation.
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3.
Practised thinking three-dimensionally about examples of polarisation.
4.
Seen that polarising filters select a preferred direction of polarisation.
5.
Seen that the direction of polarisation can be rotated by a polarising filter.
6.
Noted the use of polarisation in observing stress patterns.
Practical advice
This series of demonstrations could readily be adapted to student presentations. They are easy to
perform and students will learn a lot by having to explain them to others. The whole could be
completed in about an hour. It really helps students grasp the geometrical aspects if all the
polarising filters for each type of wave are labelled clearly with large arrows showing the
directions of permitted vibration.
Waves on rope
This should be first because it is the most visually obvious indication of what is meant by
polarisation. Refer to slots rather than 'slits' in the mechanical model or students may become
confused with diffraction phenomena.
Tie a long rubber tube to a tap or clamp at about bench height. The free end can be tensed gently
to produce a relatively slow and easily observed transverse wave. If the free end of the rubber is
waggled vertically, a vertically polarised wave is produced, which continues propagating with
vertical oscillations. It can be seen to easily pass through a vertically oriented slot in the
hardboard sheet; if, however, the slot in the polarising filter is rotated into the horizontal direction,
the incident wave is blocked (it may be absorbed or reflected). You should show what happens
for other orientations of the direction of vibration of the rubber and for the polarising filter.
Optical
1.
Place a piece of polarising filter on the overhead projector and show the reduction in
Intensity of transmitted light. Now place a second polaroid over the first with the direction
of permitted vibration parallel to the first, showing little extra reduction in intensity.
2.
Now slowly rotate the upper polariser until its direction of vibration is perpendicular to the
lower one, to show the absorption of light by crossed polaroids. If there are sufficient
pieces of polaroid let students observe light reflected from surfaces in the lab through the
filters. By rotating the filter they should be able to see that reflected light is partly
polarised (vertically from a vertical surface and horizontally from a horizontal surface).
3.
By placing a third polaroid filter between the other two at an angle of 45°, rotation of the
direction of polarisation is observed, and some component of the light is now transmitted
through the crossed polar filters.
4.
If the third filter is replaced by an injection moulded plastic ruler, coloured contours of
light are observed, showing the stress patterns in the plastic (different colours having
their directions of vibration rotated by different amounts according to internal stress in the
sample). Acetate strips can have a notch cut in them and be stressed effectively between
crossed polaroids, illustrating photoelastic stress analysis.
The permitted direction of electric oscillations for polaroid filters can be labelled by remembering
that light reflected from a vertical surface is partly polarised in the vertical direction. Rotate the
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polaroid until maximum intensity is observed in the reflected light, and then draw a vertical twoheaded arrow on the filter in a permanent marker.
Microwaves
1.
Face the transmitter towards the receiver a few metres away on the bench. Turn on the
transmitter and if possible modulate the amplitude of the signal at an audio frequency
(usually 100 Hz or 1 kHz). The receiver can be connected to an audio amplifier and
loudspeaker so that the microwave modulation signal can be 'heard'. Inspection of the
horns of the devices should suggest that the electric oscillation is vertical (see especially
the vertical diode in the receiver). If either the transmitter or receiver is rotated through
90° about the direction of propagation, the received signal drops to zero.
2.
A polarising filter for microwaves is a grille of metal wires. You can show the class that
the grille does not transmit (absorbs or reflects) if the wires are parallel to the electric
vector. At intermediate angles the polarising filter passes the component of the electric
vector that is parallel to its direction of permitted vibration.
3.
The rotation of the direction of vibration of the electric vector can be shown by crossing
the transmitter and receiver until no signal is detected. Then place the polarising filter at
say 45°. Polarising filters effectively rotate the direction of polarisation of the incident
wave, by transmitting a component of the oscillation parallel to the permitted direction.
Remember that the 3 cm wave polarising grille permits electric vector vibration (conventional
direction of polarisation) perpendicular to the wires. The energy of electric vector oscillations
parallel to the wires is absorbed by sympathetic motion of the free electrons in the metal wires.
Note that infrared has been polarised by a metal grid, this was reported by Bird and Parish; the
wire spacing must be smaller than the wavelength. [Diffraction gratings have gaps a few
wavelengths in size at least.]
(G. R. Bird and M. Parrish, Jr., “The wire grid as a near infrared polarizer,” J. Opt. Soc. Am.50,
886 (1960))
1GHz waves
You can easily show that 1 GHz radio waves have a unique direction perpendicular to their
direction of propagation, i.e. that they are polarised. The UHF transmitter emits waves polarised
parallel to the dipole direction (there is an oscillating electric field across the gap between the
dipoles).
Face the transmitter towards the receiver a few metres away on the bench and turn on the
transmitter. The receiver, which is a diode rectifier, can be connected to a sensitive light beam
galvanometer as detector. Again if either the transmitter or receiver is rotated through 90° about
the direction of propagation, the received signal drops to zero. At intermediate angles a
component of the electrical oscillation is detected.
A polariser for UHF radio waves is simply a metal rod (about 15cm long or 1/2 wavelength). It
needs to be held in a non-conducting wooden clamp. You can show the class that the rod rotates
the direction of electric oscillation by placing it at 45° between crossed transmitter and receiver.
Alternative approaches
The greater the variety of equivalent polarisation demonstrations that can be performed in the
lesson the better.
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Social and human context
The technological importance of the polarisation of electromagnetic waves in their transmission
and reception, and in aerial design. Polaroid glasses reduce glare and enable the user to see
through reflections from water or glass. Photoelasticity and the optical stress analysis of model
structures can be more economical than building prototypes. The blue light, scattered by gas
molecules and density fluctuations in the air, is partly polarised, and is used by some insects
including bees to aid their navigation.
External reference
This activity is taken from Advancing Physics chapter 3, 120P
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TAP 313 - 2: Polarisation by scattering
Scattered light is polarised
Light like that from the blue sky is scattered. It is also polarised, and the direction of polarisation
contains information about the direction of the Sun, used by birds and insects for navigation.
You will need

parallel beam projector (with small circular aperture)

power supply, 0–12 V dc and ac, 6 A

plastic tank, rectangular

polariser, optical

skimmed milk (a few drops)
Getting scattered light
Light is scattered by very small particles suspended in water. If it is scattered at right angles to its
original direction, the scattered light cannot oscillate along its direction of travel, so it must be
polarised.
Polarisation by scattering
this light can not oscillate
along its line of travel:
light scattered vertically
is polarised
unpolarised light
enters tank
light scattered horizontally
is polarised
this light can not oscillate
along its line of travel:
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1.
Fill a transparent tank with water. Add a drop or two of milk and stir. The aim is to make
the water cloudy enough for a beam of light shone through it to be just visible, no more,
from the side.
2.
Shine a narrow beam of light through the tank.
3.
Look through a polarising filter at the beam as seen looking horizontally at the side of the
tank. The diagram shows that this light must be vertically polarised. Turn the polariser
until the beam becomes invisible. The direction of polarisation in which the polariser
passes polarised light must now be parallel to the beam in the tank. Mark this direction on
the polariser.
4.
Now look down on the beam from above. This scattered light must also be polarised at
right angles to the beam, as shown in the diagram. Test this by holding the polariser with
its transmission direction along the tank: the beam should become invisible again.
You have seen
1.
Light being polarised by scattering.
2.
How to deduce the polarisation direction of a polariser.
Practical advice
This demonstration involves a neat piece of logical thinking which good students can and should
enjoy. A simple argument tells us the direction of polarisation of scattered light. Because light is
transverse, the electric field oscillates at right angles to its direction of travel. But if it is scattered
at right angles to the direction of travel, oscillations in the scattering direction cannot contribute to
light going in that direction or it would be longitudinal, not transverse.
More concretely, light in the original beam will set electrons oscillating in scattering particles. Only
the oscillations at right angles to a scattering direction can contribute to sending light in that
direction.
For the demonstration to work it is essential not to have so much scattering material (milk) that a
photon is scattered more than once. Like light from clouds in the sky, scattered light from really
milky water is not polarised; it has come from so many directions that the original direction is lost.
In practice this means that the scattered beam must be only just visible, which is achieved with
much less milk than one would anticipate. Try one drop at first, and one more if necessary.
Alternative approaches
If the day is sunny and the sky blue, take the chance to check that the blue sky at right angles to
the direction of the sun is polarised, and note that the direction agrees with that deduced from the
direction of the Sun.
Social and human context
Some insects and birds use the polarisation of light from the sky for navigation.
External reference
This activity is taken from Advancing Physics chapter 3, 130E
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TAP 313 - 3: Polarisation of light by scattering
Unpolarised light enters from the left.
Horizontally polarized light is scattered upwards; vertically polarized light is scattered horizontally.
(Diagram: resourcefulphysics.org)
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TAP 313 - 4: Polarisation in practice
Answer these questions about uses of polarisation.
Radio and television
Most portable radios have a vertically extendable stick aerial of a metre or so in length to receive
signals. TV aerials can have many short horizontal rods attached to them.
1.
What limit does the length of the stick place on the frequency band received by the radio
aerial?
2.
Explain why the orientations of the two aerials might be so different.
3.
Outline how the radio wave produces a varying potential difference across the aerial.
It is midday. The Sun is in the south, and is 56 above the horizon.
4.
Which direction should you face to observe the maximum polarisation of light scattered
from the sky?
5.
The sky on the horizon is observed through a Polaroid filter which is then rotated. What
angle with the horizontal will the filter make when maximum transmission occurs?
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6.
Ants emerging from a nest in shadow have been observed to turn their heads skywards
and to rotate them about their body axis. Can you make sense of this behaviour, given
that ants' eyes have a natural polarising filter?
7.
If you turned to face the northern horizon, with the Sun at your back, would you expect to
find any evidence of polarisation in the scattered light? Explain, possibly using a diagram.
Magnetic compasses are rather useless in polar regions. However, if the Sun is below the horizon
but there is scattered light from the sky a solar compass can be used which analyses the
polarisation of the scattered light.
8.
Why are magnetic compasses useless near the Earth's poles?
9.
How might a solar compass operate, and why would you need a clock as well to
determine direction?
The next group of questions attempt to get you to think about photoelastic stress analysis and
what might be occurring in a stressed plastic specimen showing the effect between crossed
polarising filters.
10.
What must the plastic be doing to the light to enable passage through crossed Polaroid
filters?
11.
What evidence is there that different colours have their direction of polarisation rotated by
different amounts?
12.
What determines this amount of rotation?
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13.
A ruler shows coloured bands before any stress is applied. Why is this?
14.
What does a line of equal colour (an isochrome) indicate?
15.
Sugar solution has the ability to rotate the direction of polarisation of light depending on
the concentration of the solution. Suggest how this could be used in a sugar factory to
test the strength of a sugar solution.
16.
If a liquid crystal display on a watch, calculator or laptop computer is viewed through a
polarising filter, the display can be extinguished for certain orientations of the filter. What
does this tell you about the construction of such devices?
Hints
1.
Use f = c/ λ
2.
Think about the possible polarisation direction of the electric field.
3.
Consider a free electron in the metal of the aerial.
4.
What direction is south at the North Pole?
5.
In which direction in relation to the Sun will the light from the sky show maximum
polarisation?
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Practical advice
These questions practice the geometrical thinking needed to understand polarisation, and show it
in use in a variety of ways.
Extra information for question 13, some or all of which may be shared with faster students: The
removed colour depends on the stress, being removed by a process called interference, met
soon. The interference occurs between two beams that start transversely polarised and are
refracted (birefringence) and rotated by different amounts depending on the stress. Eventually the
two beams for one colour can end up oscillating in opposite senses and cancelling to zero. A very
deep explanation of a beautiful and simple to observe phenomenon.
Alternative approaches
Some questions of your own, or possibly some home experiments on polarisation using polarising
sunglasses.
Social and human context
Exploiting the contexts implicit in the questions can lead to fruitful discussion.
Answers and worked solutions
8
1.
Around 100 MHz or greater. f > 3  10 / 2 Hz.
2.
The local radio wave is vertically polarised for its electric vector, whereas the local TV
signal appears to be horizontally polarised.
3.
The passing oscillations of the electric field exert a force on the free electrons in the
aerial and force them to vibrate in sympathy, setting up a small voltage which can be
detected and amplified.
4.
Facing either west or east. Waves scattered transverse to the original direction of
propagation can have no component of vibration parallel to that original direction.
5.
The permitted direction of the polaroid should be transverse to the line of elevation of the
sun, i.e. at 34° to the horizontal. (34° = 90° – 56°).
6.
The ants may be taking a sense of direction from the polarised light from the sky, when
the sun is not directly visible.
7.
No. Back-scattered light could have its vibration direction at any transverse angle to the
original beam direction.
8.
All directions are due south at the north magnetic pole! The magnetic field is actually
vertical over the pole!
9.
By finding the direction of maximal polarisation in skylight, a transverse direction to that
would point towards the sun. Using the time of day for expected solar position could then
yield the required orientation.
10.
The stressed plastic must be rotating the polarisation direction of the polarised light.
11.
Different colours are produced for various stresses within the specimen.
12.
The stress determines the amount of rotation, increasing the stress slowly moves the
coloured bands.
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13.
The ruler's injection moulding production process involved stressing semi-molten plastic
into a mould. As it cooled differentially from the outside inwards, stress built up in the
contracting plastic with a solid outside crust, this is locked into the material.
14.
A line of constant stress. The coloured lines are not bright spectral colours as you would
observe for white light dispersed through a prism, but rather a duller spectrum of white
light with one colour removed (as in soap bubbles).
15.
Place the solution in a standard rectangular cell between crossed polarising filters. Some
light will pass due to the rotation of the polarisation by the sugar solution. If the second
polarising filter is rotated until the light is obscured, the angle through which the filter is
rotated is a measure of the degree of rotation and hence of the strength of the solution.
(In fact these polarimeters show a good linear response. Sugar is optically active
because of asymmetry in its molecular shape – some sugars rotate to the right and some
to the left – nature's sugar molecules seem to prefer one sense of rotation!)
16.
The displays must already incorporate another polarising filter, or themselves produce
polarised light.
External reference
This activity is taken from Advancing Physics chapter 3, 140S
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TAP 313 - 5: Home experiments with radio and television signals
Just take a look
You can learn quite a lot about radio waves by having a look at radios and television sets and
their aerials.
Safety
Do not to delve into high-voltage equipment or to climb on roofs to get a
better look at aerial
You need access to some of these

TV set with indoor aerial

TV with aerial on the roof

FM radio with aerial on roof

portable radio, preferably AM and FM
Radio spectrum
Do you have a portable FM radio with dial rather than push-button tuning? If so, spin the dial and
notice the frequencies at which stations come up. Typical frequencies are in the range 90–100
MHz or so. The strong signals will not be closer than 0.2 MHz apart. Notice the range of
frequencies over which you can still hear a strong signal as you tune the radio 'through' its
frequency. It may be about 0.1 MHz either side of the correct frequency. That 'bandwidth' allows
for the variations in frequency produced by the radio waves carrying the audible signal.
You may get less strong stations, generally ones meant to be picked up somewhere else, very
close to the frequency of one of your local stations.
Does your FM radio have push-button tuning? If so, there is probably a Search button which runs
up or down the frequency scale, stopping when the radio detects a signal. Use this to find the
frequencies of stations as above. If there is a Fine Tuning control, try that to see the spread of
frequencies over which you can still hear a station.
If you have an AM radio, try locating stations on the medium wave band. How does their quality
compare to that of FM stations? Again, over what range of frequencies can you tune and still hear
the station? How does it compare to the range for FM? Are there examples of stations too close
together which you hear one on top of the other?
Finally, calculate the wavelengths for some FM and AM stations.
Indoor and outdoor aerials
Do you have a TV with an indoor aerial? It may be a loop of wire, or it may be some metal rods.
Measure the approximate dimensions of the aerial – the length of the rods or the diameter of the
loop. Working on the rule of thumb that the wavelength is often about twice the dimensions of the
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aerial, estimate the frequency of TV signals. Does your estimate put the frequency in the UHF
band 300 MHz to 3 GHz?
Turn the aerial around with the TV switched on. Does the picture vanish or get faint in any
direction? Is it best in some other direction? You may be able to guess the direction of the TV
transmitter from this.
Do you have a TV aerial on the roof, or can you see someone else's roof aerial? Look for
something like this:
this mesh reflects the signal back
onto the one in front of it. It may
be a square wire mesh or a grid of
rods instead
this rod is the one which actually
picks up the signal, and is
connected to the reciever
these rods make the aerial more
sensitive and more directional
about half a
wavelength
If there is more than one such aerial, the one for TV is the one with the shortest rods.
The rods may be vertical or horizontal. The aerial points towards the TV station. Estimate the
length of the rods, top to bottom (look for something like chimney bricks whose size you know to
guess the length).
Use the rule of thumb that the wavelength is about twice the full length of the rods, to estimate the
wavelength. Does your estimate put the frequency in the UHF band of 300 MHz to 3 GHz?
Now look for FM radio aerials on the roof. They look like the TV aerials but the rods are longer
and there are usually only a few of them. Again estimate the wavelength from a guess at the
length of the rods, and see if that puts the frequency in the FM band around 90 to 100 MHz. At
least notice the difference in size between TV and FM radio aerials, because of the difference in
wavelength.
Is there a TV satellite dish on a house near you which you can get a look at? – No need to touch.
Offset from the centre of the dish is the collecting aerial, inside a covered tube. Knowing that
satellite frequencies are somewhere in the SHF 3 GHz to 30 GHz band, what wavelengths might
they have? How many times bigger than the wavelength is the dish diameter? Why would that
be?
Also, think about where the satellite must be, having noticed that the collecting aerial is not quite
on the axis of the dish.
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Not all aerials are of the order of a wavelength in size. Mobile telephones have short stubby
aerials yet use frequencies in the region of 900 MHz, wavelength around 300 mm.
Polarisation
Look again at TV and FM aerials on a roof, if you can see any. Notice whether the rods are
vertical or horizontal.
horizontal position
vertical position
The aerials with vertical rods are for stations that send out electromagnetic waves polarised
vertically. The horizontal rods detect waves that are polarised horizontally. This can be used to
stop stations getting in each other's way if they use frequencies close together, since one aerial is
bad at detecting signals with the opposite polarisation. Next time you take a rail trip; look at the
aerials on roofs in big towns some distance apart. Are they lined up for opposite polarisations?
You may be able to use an indoor TV rod or loop aerial to detect polarisation too. Find the station
direction by pointing the aerial to get a good signal, and then turn it around that axis. Does the
picture get better or worse?
Waves adding together
It's common with an indoor TV aerial to find that the picture depends on where the aerial is in the
room, and where people are near it. Try putting the aerial down and walking around it. Does the
picture come and go? If so, notice how far you move between places where the picture is good
and bad. The waves bounce off your body and add to those going direct to the aerial. This can
make the picture better if the direct and reflected waves are in step when they get to the aerial.
But if they are out of step, peak to trough and trough to peak, they add together to give less than
either alone. With waves, more can be less.
If you have a portable FM radio and are near a large wire fence (perhaps round a school or park)
you can try and see whether you can get the FM signals reflected from the fence to enhance or
reduce the signal at the radio. Try walking towards and away from the fence, carrying the radio.
Don't expect any effect for movements of less than about half a wavelength, say around 1.5
metres.
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You may have seen
1.
That radio stations are spaced along the radio spectrum, each occupying a certain width
of the radio band.
2.
That the size of many aerials tells you the approximate wavelength involved.
3.
That the orientation of an aerial can tell you the direction of polarisation of the radio wave.
4.
That two or more waves can sometimes combine to make less, not more.
Practical advice
These activities are suggested mainly to link students' everyday observations with their work in
physics. You may wish to follow up this work with a class discussion. It will be important to be
clear that not all the suggestions can be followed up. In some places, there are few external
aerials, for example. The coming of cable will certainly reduce their number. Also, a top quality
FM radio is very good at preventing you hearing the effects of bandwidth or of noticing fading as
waves are reflected onto the aerial, because it has automatic frequency and volume control. The
result is that poor signals are made as good as possible until they are lost. So encourage the use
of the cheapest portable radio possible!
Safety
Do not to delve into high-voltage equipment or to climb on roofs to
get a better look at aerial
Alternative approaches
You may wish to import some of these experiments into the classroom. Also, it may be possible
to devise observations using mobile 'phones, though these will be unpopular if they require long
expensive connections or interrupt the calls.
Social and human context
Technology has a tendency to be taken for granted. It is also often designed to be easy to use but
hard to see how it works.
External reference
This activity is taken from Advancing Physics chapter 3, 110H
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TAP 313 - 6: Polarimetry
Use a polarimeter to measure the rotation of the plane of polarisation for some sugar solutions,
and hence explain how polarimetry can be used to measure sugar concentration
Apparatus

sugar solutions

polarimeter (simple type)

power supply

leads (2)

ruler

felt-tip pen

optional: samples of golden syrup and clear honey
Switch on the lamp beneath the polarimeter and then, with the containing tube in place, but
empty, rotate the top Polaroid (the analyser) until the view through the instrument is as dark as
possible. Note the position of the analyser on the scale. Pour a sugar solution into the tube to a
depth of 10 cm and replace it in the instrument. There may be a black tube or cover to place over
this sample tube to increase contrast. Look through the analyser and rotate it in a clockwise
direction until the view is again as dark as possible. (It sometimes appears dark blue at this
stage.) Note the new position of the analyser. The position of maximum darkness is not usually
very definite.
Record the range of angles over which the darkness appears greatest.
Repeat for solutions of various concentrations, including at least one unknown concentration.
Record you results in a suitable table, and calculate the rotation angle for each solution together
with its uncertainty.
Plot a calibration graph, complete with error bars, to show how the rotation angle varies with
concentration.
Use your graph to determine the concentration of your unknown solution(s).
If you have a little spare time you could also note the effect of changing the depth of solution. Try
5 cm and 2.5 cm.
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Practical advice
In order to show the general idea of polarimetry, you might like to show students what happens
when sugar solution is placed between crossed Polaroids before they embark on the work of this
section. During this part of the teaching unit, you might like to show the video `Polarisation of
Light' (see details below).
Although the student activity will show that the rotation is directly proportional to the
concentration, more accurate measurements show that this is not exactly the case. The rotation
is also dependent on temperature and, to a slight extent, on the solvent used.
Professional polarimeters usually use sodium light, with a wavelength of 589 nm. The student
instrument may be slightly improved by placing a filter between the bulb and the polariser, or by
using light from a sodium lamp.
Typical results are shown in below. A positive angle denotes a clockwise rotation, i.e. a
dextrorotatory material, and a negative angle denotes rotation in the opposite sense. A
demonstration with honey and syrup illustrates the two senses of rotation.
Technician’s notes
The video `Polarization of Light' may be shown in this teaching unit. It is available from:
Classroom Video, Unit 2, Northavon Business Centre, Dean Road, Yate, Bristol BS37 5NH
Phone: 01454 324222, Fax: 01454 325222, e-mail: orders@classroomvideo.co.uk
Web: http://www.classroomvideo.com/Home/default.aspx
Use commercial granulated sugar and distilled water to make up the following solutions, labelled
as indicated:
25 g sugar in 100 ml water, labelled `20% by mass'
50 g sugar in 100 ml water, labelled `33% by mass'
75 g sugar in 100 ml water, labelled 43% by mass'
37.5 g sugar in 100 ml water, labelled `solution A'
62.5 g sugar in 100 ml water, labelled `solution B'
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The distilled water should be labelled `0% by mass'.
The solutions should ideally be made up the day before and left so that they are all at room
temperature.
Your chemistry department might have a polarimeter that you can borrow; check before buying
one, as they are expensive. Details of how to make a simple polarimeter are given in the Revised
Nuffield Advanced Chemistry Teachers' Guide 2, pp. 203-205, ISBN. 0582353637.
Depending on the type of polarimeter available, some screening from direct light may be required.
Some models do this by having the container within a sealed box, others provide a cover, yet
others have nothing at all.
If using honey and syrup, pour them into the polarimeter tubes the night before, to allow time for
air bubbles to escape.
External reference
This activity is taken from Salters Horners Advanced Physics, section GETE, activity 21
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